The Drying-Rack Bound: Mould-Risk Windows for Sun-Dried Chillies on the Ground, on a Raised Rack, and Under a Solar Cover

Chillies harvested at 80 percent moisture must reach about 10 percent before they are safe to store; between those figures they pass through a moisture band — above 14 percent wet basis, water activity above roughly 0.70 — in which moulds grow at ordinary warm temperatures. This paper computes, from stated drying behaviour, how long chillies spend in that band under three open-air methods (spread on the ground, spread on a raised rack at 0.9 m, and spread under a simple solar cover) and three stated spells of weather (a fair spell at 30 °C and 55 percent relative humidity, a humid spell at 26 °C and 80 percent, and a wet spell at 24 °C and 90 percent), against a stated risk criterion of 72 accumulated hours in the band. In the fair spell the solar cover finishes in 2.12 days with 33 hours in the band, the only method inside the criterion; the rack finishes in 4.21 days but accumulates 77 band hours, and the ground layer needs 8.29 days and 146 band hours, so both open methods breach the criterion even in good weather. Leaving produce out at night costs the ground layer a full extra day (8.29 days against 7.29 when it is carried into a dry store nightly) but costs the rack and the cover only about 0.04 days, because whole pods re-wet slowly and raised produce re-wets slowest. In the humid spell the ground and rack methods never finish at all — the equilibrium moisture of the air, 18 percent, lies above the safe target — stalling near 20.7 and 19.3 percent after 30 days, while the solar cover finishes in 5.17 days but still breaches the criterion at 75 band hours. In the wet spell none of the three methods dries the crop safely: the cover stalls at 14.8 percent, still inside the mould band, and the open methods stall at 24.0 and 22.8 percent. The rack-height rule the arithmetic supports is to build at 0.6 to 0.9 m — raising a rack from 0.15 m to 0.9 m cuts band exposure from 103 to 77 hours, while the step to 1.2 m buys only one further hour. This study is simulated, not measured.